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Flexing Nano-muscles

Tony Huang makes motors out of molecules.

Tom Imerito

November 26, 2007

Flexing Nano-muscles

Tony Huang's enthusiasm for nanoscience is both obvious and irrepressible. "When nanotechnology came around, sometimes I couldn't even sleep at night, I was so excited about the problems I was working on," he says. "Even today, every day when I come to my office, I am very excited about some new idea."

Trained as a mechanical engineer in his native China, Huang once worked on the fluid mechanics of advanced nuclear power cooling systems, plausibly the largest end of the engineering spectrum, where things are measured in meters and tons. Today, he works at the smallest end of the spectrum, investigating objects and events that are measured in nanometers and atomic weights.

To Huang, the shift in scale is almost immaterial. "Eighty percent of our body is fluid," he explains. "And fluid mechanics problems cannot be solved by biologists or chemists. Only a mechanical engineer can solve them."

Courtesy Tony Huang

Schematic diagram of an artificial-molecular-machine-based mechanical device developed by nano-engineer Tony Huang.

In 2006, Huang and colleagues Chung-Chiun Liu of Case Western Reserve, Fraser Stoddart of UCLA and William Goddard of Caltech were awarded a four-year, $1.3 million grant from the National Science Foundation to investigate a class of polymers called rotaxanes. "We are studying the electrical, optical and mechanical properties of these systems, from nanometer scale to micrometer to macrometer scale," Huang said.

Rotaxane's appeal as a bio-mechanical material stems from its ability to cross the boundaries of chemistry and physics at nanoscale. As Huang explains it, rotaxane's molecular character gives it an unusual ability to form mechanical associations, in which one molecular component can be physically trapped within or around another component without being chemically bound to it. In its simplest form, the rotaxane molecule consists of a molecular ring riding on a molecular bar with molecular stoppers at each end to keep the ring from sliding off. Graphic representations look something like a dumbbell. Depending upon the chemical compositions of the system's various components, the molecule can be designed in a variety of configurations.

Huang and his colleagues have already designed molecular motors with rotaxane. As with the molecule described above, molecular rings shuttle back and forth on a molecular bar in response to changes in the electrochemical charge of the solution in which they reside. As Huang explains, oscillation of the charge by reciprocal oxidation and reduction (a redox reaction) causes the rings to alternately gain and lose electrons, which attracts or repels them to and from each other and the molecular stoppers.

But since a mass of molecules sloshing in a fluid does not constitute useful work, the group has developed a technique for attaching to the rings a sulfur-based molecular tether that tends to anchor itself to gold. By depositing a thin layer of gold on a silicon substrate, they can thereby attach their tiny motors to a flexible cantilever that deflects upward or downward in response to the rotaxane rings' tugging on the anchored tethers, Huang says.

When configured in an array of flexible microcantilever beams, each coated on one side with a monolayer of six billion rotaxane molecules, he adds, the cantilevers undergo controllable and reversible bending up and down when exposed to chemical oxidants and reductants. The aggregate force of the molecules is harnessed and put to work very much the way the tiny filaments in the muscles in our bodies work together to transfer molecular force to our bones, Huang says.

Citing the near-term potential of these molecular motors, Huang says, "We use these motors to drive fluid like a pump. These days, microdevices are small but the power supply required is huge—today's micropumps require fifteen to twenty volts. These pumps require only .5 to 2 volts. That will help a lot of different applications."

Looking toward the day when in-vivo DNA recognition is a routine part of our healthcare system, Huang speculates, "If a single mutation is detected, maybe we will be able to send a molecular motor to that DNA site and splice the correct DNA. We want to develop some type of device that will someday benefit peoples' lives."

Tony Jun Huang, Ph.D., is James Henderson assistant professor of Engineering Science and Mechanics in the College of Engineering and director of Penn State's Bio-NEMS (Biofunctionalized Nano-Electro-Mechanical-Systems) group, junhuang@psu.edu. The rotaxane work described above is funded by a four-year, $1.3 million NIRT (Nanoscale Interdisciplinary Research Teams) grant from the National Science Foundation.